This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a graded thermal/environmental barrier coating system for a substrate formed of a material containing silicon.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. Materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material and/or as a reinforcing material, are currently being considered for high temperature applications, such as combustor and other hot section components of gas turbine engines.
In many applications, a protective coating is beneficial or required for the Si-containing material. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings should provide environmental protection by inhibiting the major mechanism for degradation of Si-containing materials in a corrosive water-containing environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, besides low thermal conductivity, a critical requirement of a thermal barrier coating system for a Si-containing material is stability in high temperature environments containing water vapors. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with the Si-containing material and silica scale formed from oxidation. As a result, suitable protective coatings for gas turbine engine components formed of Si-containing materials essentially have a dual function, serving as a thermal barrier and simultaneously providing environmental protection. A coating system having this dual function is termed a thermal/environmental barrier coating (T/EBC) system.
While various single-layer and multilayer T/EBC systems have been investigated, each has exhibited shortcomings relating to the above-noted requirements and properties for compatibility with a Si-containing material. For example, a coating of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibits excellent environmental resistance by itself, since it does not contain silica in its composition. However, YSZ does not adhere well to Si-containing materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/xc2x0 C. for YSZ as compared to about 4.9 ppm/xc2x0 C. for SiC/SiC composites). Mullite (3Al2O3xc2x72SiO2) has been proposed as a bond coat for YSZ on Si-containing substrate materials to compensate for this difference in CTE (mullite having a CTE of about 5.5 ppm/xc2x0 C.). However, mullite exhibits significant silica activity and volatilization at high-temperatures if water (water vapor) is present.
Barium-strontium-aluminosilicate (BSAS) coatings suitable for Si-containing materials exposed to temperatures of up to 2400xc2x0 F. (about 1315xc2x0 C.) have also been proposed. BSAS provides excellent environmental protection and exhibits good thermal barrier properties due to its low thermal conductivity. However, for application temperatures approaching the melting temperature of BSAS (about 1700xc2x0 C.), a BSAS protective coating would require a thermal-insulating top coat. BSAS has been proposed as a bond coat for YSZ in U.S. Pat. No. 5,985,970 to Spitsberg et al., assigned to the assignee of the present invention. The inclusion of a BSAS bond coat significantly increases the overall thickness of the T/EBC system. As application temperatures increase further beyond the thermal capability of a Si-containing material (limited by a melting temperature of about 2560xc2x0 F. (about 1404xc2x0 C.) for silicon), still thicker coatings capable of withstanding higher thermal gradients are required. However, as coating thickness increases, strain energy due to the CTE mismatch between individual coating layers and the substrate increases as well, which can cause debonding and spallation of the coating system.
Accordingly, there is a need for a thick T/EBC system for Si-containing materials that enables such materials to be used at application temperatures beyond the melting temperature of silicon.
The present invention generally provides a coating system for Si-containing material, particularly those for articles exposed to high temperatures, including the hostile thermal environment of a gas turbine engine. Examples of such materials include those with a dispersion of silicon carbide, silicon carbide and/or silicon particles as a reinforcement material in a metallic or nonmetallic matrix, as well as those having a silicon carbide, silicon nitride and/or silicon-containing matrix, and particularly composite materials that employ silicon carbide, silicon nitride and/or silicon as both the reinforcement and matrix materials (e.g., SiC/SiC ceramic matrix composites (CMC)).
The invention is a compositionally-graded thermal/environmental barrier coating (T/EBC) system that exhibits improved mechanical integrity for high application temperatures that necessitate thick protective coatings, generally on the order of 250 xcexcm or more. The T/EBC system includes an intermediate layer containing YSZ and mullite, alumina and/or an alkaline earth metal al minosilicate (preferably BSAS), which is preferably used in combination with a mullite-containing layer that overlies the surface of the Si-containing material, a layer of an alkaline earth metal aluminosilicate (again, preferably BSAS) between the mullite-containing layer and the intermediate layer, and a thermal-insulating top coat of YSZ overlying the intermediate layer. Particular embodiments are for the intermediate layer to have a substantially uniform composition of YSZ and either BSAS, mullite or alumina, or to contain sublayers within an innermost sublayer (contacting the BSAS layer) being BSAS, mullite or alumina and an outermost sublayer (contacting the YSZ top coat) being YSZ, or to be compositionally graded so that the concentrations of YSZ and BSAS, mullite or alumina continuously change through the thickness of the intermediate layer.
The mullite-containing layer has a CTE above that of a Si-containing substrate but less than that of the YSZ top coat, and therefore compensates for the difference in CTE between the Si-containing substrate and the other coating layers. In addition, the mullite-containing layer serves as a chemical barrier between BSAS layer and the Si-containing substrate to prevent interaction of BSAS with the silicon oxidation product (SiO2) at high temperatures. The BSAS layer provides environmental protection to the silicon-containing substrate. The top coat of YSZ offers excellent thermal protection to the Si-containing substrate and the other underlying layers of the coating system. Finally, the YSZ-containing intermediate layer serves as a thermal barrier layer that also provides a CTE transition between the BSAS layer and the YSZ top coat as a result of its BSAS, mullite and/or alumina content, each of which has a CTE between that of YSZ and Si-containing materials.
According to this invention, a compositionally-graded T/EBC as described above is able to reliably provide both thermal and environmental protection to a Si-containing substrate at temperatures of up to 2000xc2x0 C., particularly when present at total coating thicknesses of 250 xcexcm or more, as a result of exhibiting improved mechanical integrity as compared to prior art coating systems for Si-containing materials.
Other objects and advantages of this invention will be better appreciated from the following detailed description.